1. Field of the Invention
The present invention relates generally to a polishing machine for polishing a wafer, and more particularly to a polishing machine which is able to uniformly polish the whole surface of the semiconductor wafer.
2. Description of Related Art
As semiconductor devices have recently become larger scale and consisted of multiple layers, it has become more important to accurately plane a semiconductor wafer in the process of manufacturing the semiconductor devices. To plane the semiconductor wafer, polishing liquid is supplied to an area between the semiconductor wafer and a polishing pad, and the semiconductor wafer and the polishing pad are moved relatively to each other and pressed against each other so that the semiconductor wafer can be polished.
In order to accurately polish the semiconductor wafer, the semiconductor wafer and the polishing pad must be maintained precisely parallel, and further, a pressure between the semiconductor wafer and the polishing pad (hereinafter referred to as a polishing pressure) must be uniformly applied to the whole surface of the semiconductor wafer.
In the conventional semiconductor wafer polishing machine, only the semiconductor wafer is pressed against the polishing pad so that the semiconductor wafer can be polished, and hence an elastic stress of the polishing pad concentrates on the edge of the semiconductor wafer. That is, the polishing pressure concentrates on the edge of the semiconductor wafer, and thereby only the edge of the semiconductor wafer is excessively polished. In addition, the semiconductor wafer sinks in the surface of the polishing pad, and the semiconductor wafer and the polishing pad are inclined relatively to each other as a result. Consequently, the semiconductor wafer cannot be uniformly polished.
On the other hand, in order to uniformly polish the wafer, the polishing liquid is preferred to be uniformly distributed over the area between the semiconductor wafer and the polishing pad. In the conventional machine, the polishing liquid is dropped on the surface of the polishing pad so that the polishing liquid can infiltrate the area between the semiconductor wafer and the polishing pad. The dropped polishing liquid infiltrates some extent at the periphery of the semiconductor wafer; however, the polishing liquid cannot easily infiltrate the center of the semiconductor wafer. Hence, there is a difference in the quantity of the supplied polishing liquid between the center of the semiconductor wafer and the periphery thereof, and the semiconductor wafer cannot be uniformly polished.
Moreover, the polishing pad is generally made of porous material so as to hold the dropped polishing liquid. If pores on the surface of the polishing pad are clogged up with polish-dust and abrasive grains in the polishing liquid, the polishing efficiency will be lowered considerably. Accordingly, the dressing must be performed at regular intervals so as to remove the polish-dust and abrasive grains which clog up the pores.
In the conventional machine, the polishing stops every time the dressing starts, and the throughput cannot be improved as a result. Further, special equipment is required to perform the dressing.